For an undertread, low heat build-up is an important physical property since the undertread is arranged in the tire interior, and thus the undertread readily builds up heat.
Furthermore, if the elastic modulus of the undertread is small, motion of adjacent parts becomes larger and the heat build-up of the tire may be deteriorated as a result. Therefore, elastic modulus needs to be fully considered.
In response to such required characteristics, for example, Patent Document 1 describes “a rubber composition for a tire undertread, the rubber composition comprising, per 100 parts by mass of a diene rubber, from 10 to 40 parts by mass of an oil-extended white clay, 15 parts by mass or greater of a carbon black, and from 1 to 10 parts by mass of a silica, a total compounded amount of the oil-extended white clay, the carbon black, and the silica being from 40 to 70 parts by mass” ([Claim 1]).